In many optical systems, the lenses, objectives or other optical elements that direct light onto a surface and/or receive light back from a surface are essentially fixed with respect to the object that is being imaged or analyzed. The only movement that is typically provided for is that which is required to properly focus the optical system on the object. Given the high resolution of most modern optical systems, this amount of movement is quite small. One skilled in the art will appreciate that the amount of travel of an optical system that is required for focusing is dependent on the resolution of the optical system.
In some applications the variation in topography of the substrate that is being analyzed is sufficiently large so that some of the features are out of the focus range of the optics at any set distance between the optics and the substrate. Some optical systems have been developed to accommodate this scenario by raising and lowering the substrate relative to the optics during the optical inspection of the substrate. Repeatedly raising and lowering the substrate can cause damage to the substrate and/or result in slow and inaccurate and inefficient inspection thereof, and can also be very difficult if not impossible to accomplish depending on the size and nature of the substrates. There is a need for improved optical systems and methods that are able to inspect substrates that have relative large variations in topography.